This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to a Japanese Patent Application 2001-083100 filed on Mar. 22, 2001, the entire content of which is incorporated herein by reference.
This invention generally relates to an oil pressure control apparatus for an automatic transmission. More particularly, this invention pertains to an oil pressure control apparatus for an automatic transmission for changing a shift stage by selectively supplying control pressure outputted from plural control valves to a required friction engagement element.
A known oil pressure control apparatus for an automatic transmission switches a shift stage from a neutral shift stage to a first shift stage by selectively supplying control pressure outputted from plural control valves to a required friction engagement element. Assuming the neutral shift stage is performed by a first combination of the friction engagement elements to be engaged, the control pressure supplied from each control valve to each friction engagement element is set to be approximately at a maximum oil pressure level to maintain the friction engagement elements for the first combination under an engaged condition.
Assuming the first shift stage is performed by a second combination of the friction elements to be engaged upon a shifting operation of the shift stage from the neutral shift stage to the first shift stage, a hydraulic circuit of the oil pressure control apparatus is switched for supplying the control pressure from each control valve to each friction engagement element for the second combination while the friction engagement elements for the first combination have been maintained under the engaged condition by continuously supplying the control pressure from the control valves thereto. The control pressure supplied to the friction engagement elements for the second combination is then gradually increased and the control pressure supplied to the other friction engagement elements is gradually decreased. Therefore, the friction engagement elements for the second combination are gradually shifted to the engaged condition and the other friction engagement elements are gradually shifted to a disengaged condition to smoothly shift the shift stage from the neutral shift stage to the first shift stage.
The control pressure supplied to the friction engagement elements for the second combination is then set to be approximately at a maximum oil pressure level to maintain the friction engagement elements for the second combination under the engaged condition. On the other hand, the control pressure supplied to the other friction engagement elements is set to be approximately at a minimum oil pressure level to release the friction engagement elements from the engaged condition. Under the above described condition, the first shift stage is continuously performed.
However, the above described oil pressure control apparatus may suffer from the following drawbacks. The engaging and disengaging operations of the required friction engagement elements are performed only by increasing or decreasing the control pressure to be supplied to the elements from the control valves. In this case, the engaging force for engaging the friction engagement elements is required to have a certain amount of engaging force when the first shift stage with a large torque is continuously performed. Therefore, a maximum pressure level of the control pressure is required to be sufficiently large for engaging the friction engagement elements.
However, when the shift stage is shifted to the first shift stage as described above, the control pressure supplied to the required friction engagement elements is required to be gradually increased. In this case, the adjusting range of the control pressure will be increased when the maximum oil pressure level of the control pressure is set to be sufficiently large based upon the engaging force of the friction engagement elements to be engaged for continuously performing the first shift stage. Therefore, the pressure adjustment ability may be deteriorated and the shifting operation may not be accurately performed. On the other hand, in case the maximum pressure level of the control pressure is restrained by heightening the resolving ability of the pressure adjustment, the friction engagement elements to be engaged for the first shift stage may slip when the torque is increased in response to a depressing operation of the accelerator pedal. Therefore, the friction engagement elements may not be maintained under the engagement condition.
Accordingly, the above-mentioned automatic transmission oil pressure control apparatus is still susceptible of certain improvements with respect to ensuring sufficient oil pressure to be supplied to the friction engagement elements for performing at least any one of the first shift stage or a reverse shift stage. Further, the apparatus is susceptible of certain improvements with respect to preferably performing the pressure adjustment of the oil pressure supplied to the friction engagement elements upon the shifting operation of the shift stage from the neutral shift stage to at least any one of the first shift stage and the reverse shift stage.
An oil pressure control apparatus for an automatic transmission including an output shaft is provided with plural linear solenoid valves, plural control valves, plural friction engagement elements, plural shift valves, and plural ON-OFF solenoid valves. The plural linear solenoid valves output modulated pressure in response to electric current applied to the linear solenoid valves. The plural control valves output control pressure in response to the modulated pressure from the linear solenoid valves. The plural friction engagement elements are engaged or disengaged in response to the supply of either the control pressure from the control valves or the line pressure for performing plural shift stages through engagement and disengagement of combinations of the plural friction engagement elements. The plural shift valves change the friction engagement element supplied with the control pressure or the line pressure in response to oil pressure applied to the shift valves. The plural ON-OFF solenoid valves change the supply condition of oil pressure to the shift valves in response to electric current applied to the ON-OFF solenoid valves.
The oil pressure control apparatus for the automatic transmission is further provided with a control unit for controlling the control pressure from the control valves by controlling the electric current applied to the linear solenoid valves and for changing the friction engagement element to be supplied with the control pressure or the line pressure via the shift valve by controlling the electric current applied to the ON-OFF solenoid valves. The control unit consequently sets a first shift pattern for supplying the control pressure to the friction engagement element for a first shift stage or a reverse shift stage from the control valve by a first combination of the current applied condition of the plural ON-OFF solenoid valves upon performing the first shift stage or the reverse shift stage and a second pattern for supplying the line pressure to the friction engagement element for the first shift stage or the reverse shift stage by a second combination of the current applied condition of the plural ON-OFF solenoid valves upon performing the first shift stage or the reverse shift stage.
Therefore, when the shift stage is shifted from the neutral shift stage to either the first shift stage or the reverse shift stage, pressure adjustment can be performed accurately by setting the first shift pattern, and the oil pressure to be supplied to the friction engagement element can be sufficiently ensured by setting the second shift pattern.
According to another aspect of the present invention, the control unit can determine the control pressure to be supplied to the common friction engagement element to be engaged at the first shift stage and at the reverse shift upon a neutral range so as to shift to the first shift stage and the reverse shift stage.
When the shift stage is shifted to either the first shift stage or the reverse shift stage by changing the shift range to a driving range or a reverse range, the oil pressure is required to be supplied only to a remaining friction engagement element which was not engaged upon the neutral range for engaging the remaining friction engagement element. Therefore, the response of the friction engagement elements can be improved by reducing the number of friction engagement elements to be engaged at one time, i.e., by reducing the number of friction engagement elements to be supplied with the oil pressure at one time.
According to another aspect of the present invention, the control unit determines the control pressure to be supplied to the required friction engagement element upon a neutral shift stage of a driving range for engaging the required friction engagement element so as not to rotate the output shaft of the automatic transmission.
Generally, the engaging operations of the required friction engagement elements are respectively interrupted so as not to transmit rotation of an engine to the automatic transmission upon the neutral shift stage of the driving range. The automatic transmission is hence required to be engaged with the engine when the vehicle is started. For example, the vehicle is moved in a vehicle rearward direction until the automatic transmission is connected to the engine again upon a hill start of the vehicle. On the other hand, upon the neutral shift stage of the driving range, the output shaft of the automatic transmission is designed so as not to be rotated as described above. Therefore, the shift stage can be shifted to the first shift stage without moving the vehicle in the vehicle rearward direction. That is, a smooth hill start of the vehicle can be performed.
According to another aspect, the supply of the oil pressure to any one of the friction engagement elements to be engaged for a vehicle forward movement can be performed via a manual valve only upon the driving range and the supply of the oil pressure to the required friction engagement elements to be engaged for a vehicle rearward movement can be performed via the manual valve only upon the reverse range.
The undesirable oil pressure supply to any one of the friction engagement elements to be engaged for the vehicle rearward movement can be prevented upon the driving range. The undesirable oil pressure supply to any one of the friction engagement elements to be engaged for the vehicle forward movement can be prevented upon the reverse range. Therefore, unintended driving upon each shift range due to mechanical failure including various valves, for example the reverse movement upon the driving range, the forward movement upon the reverse range, and the reverse and forward movements upon the neutral range, can be prevented.
According to another aspect, the plural friction engagement elements include a first friction clutch, a second friction clutch, a third friction clutch, a first friction brake, and a second friction brake. The control unit consequently sets a first shift pattern and a second shift pattern for performing a first shift stage. Upon the first shift pattern, the control pressure is supplied from the control valve to the first friction clutch to be engaged for performing the first shift stage. Upon a second shift pattern, the line pressure is supplied to the first friction clutch.
The control unit consequently sets the first shift pattern and the second shift pattern for performing a reverse shift stage. Upon the first shift pattern, the control pressure is supplied from the control valve to the third friction clutch to be engaged for performing the reverse shift stage. Upon the second shift pattern, the line pressure is supplied to the third friction clutch. Therefore, when the shift stage is shifted from the neutral shift stage to either the first shift stage or the reverse shift stage, pressure adjustment can be performed accurately by setting the first shift pattern and the oil pressure to be supplied to the friction engagement elements can be sufficiently assured by setting the second shift pattern.
Further, the control unit determines the control pressure to be supplied to the second friction brake to be engaged at the first shift stage and at the reverse shift stage for engaging the second friction brake upon the neutral range of the driving range so as to perform the first shift stage and the reverse shift stage.
When the shift stage is shifted to either the first shift stage or the reverse shift stage by changing the shift range to a driving range or a reverse range, the oil pressure is required to be supplied only to a remaining friction engagement element which was not engaged upon the neutral range for engaging the remaining friction engagement element. Therefore, the response of the friction engagement element can be improved by reducing the number of friction engagement elements to be engaged at one time, i.e., by reducing the number of the friction engagement elements to be supplied with the oil pressure at one time.
Further, the control unit determines upon the neutral shift stage at the driving range of the shift range the control pressure to be supplied to the first and second friction brakes for engaging the first and second friction brakes so as not to rotate the output shaft of the automatic transmission.
The automatic transmission is hence required to be engaged with the engine when the vehicle is started. For example, the vehicle is moved in a vehicle rearward direction until the automatic transmission is connected to the engine again upon a hill start of the vehicle. On the other hand, upon the neutral shift stage of the driving range, the output shaft of the automatic transmission is designed to not be rotated as described above. Therefore, the shift stage can be shifted to the first shift stage without moving the vehicle in the vehicle rearward direction. That is, a smooth hill start of the vehicle can be performed.
Further, the supply of the oil pressure to the first friction clutch or the second friction clutch to be engaged for a vehicle forward movement can be performed via the manual valve only upon the driving range and the supply of the oil pressure to the third friction clutch and the second friction brake to be engaged substantially at one time for a vehicle rearward movement can be performed substantially at one time via the manual valve only upon the reverse range.
The undesirable oil pressure supply to any one of the friction engagement elements to be engaged for the vehicle rearward movement can be prevented upon the driving range. The undesirable oil pressure supply to any one of the friction engagement elements to be engaged for the vehicle forward movement can be prevented upon the reverse range.
Therefore, unintended driving upon each shift range due to mechanical failure including various valves, for example the reverse movement upon the driving range, the forward movement upon the reverse range, and the reverse and forward movements upon the neutral range, can be prevented.